The following discussion of the prior art is provided to place the invention in an appropriate technical context and enable the advantages of it to be more fully understood. However, it should be appreciated that any discussion of the prior art throughout the specification should not be considered as an express or implied admission that such prior art is widely known or forms part of common general knowledge in the field.
Iron is crucial for maintaining normal structure and function of virtually all mammalian cells (see, for example, Voest et al., in Ann. Intern. Med., 120:490-499 (1994) and Kontoghiorghes, G. J., in Toxicol. Letters, 80:1-18 (1995)). Iron and its binding proteins have immunoregulatory properties, and adult humans contain 3-5 g of iron, mainly in the form of haemoglobin (58%), ferritin/haemosiderin (30%), myoglobin (9%) and other haem or non-haem enzyme proteins (Harrison and Hoare, in Metals in Biochemistry, Chapman and Hall, New York, 1980). Approximately 10 to 15 mg of dietary iron is normally consumed per day by each individual in the U.S. About 1 to 2 mg of iron in the Fe (II) form is absorbed each day chiefly through villi in the duodenum to compensate for the 1 to 2 mg daily body loss of iron. Normal men absorb about 1 mg iron per day, menstruating women 2 mg iron per day, and haemochromatosis patients 2 to 5 mg iron per day.
Total iron levels in the body are regulated mainly through absorption from the intestine and the erythropoietic activity of the bone marrow. Upon absorption, iron is transported to various tissues and organs by the serum protein, transferrin. Once transported to the target tissue or organ, iron is transported and stored intracellularly in the form of ferritin/haemosiderin. Under normal conditions, transferrin is about 30% saturated with iron in healthy individuals, and an equilibrium is maintained between the sites of iron absorption, storage and utilization. The presence of these homeostatic controls ensures the maintenance of physiological levels of not only iron, but also other essential metal ions such as copper, zinc and cobalt. The control of iron absorption may be genetic with complex interactions with intestinal mucosal cells, dietary factors, and other influences.
Iron is absorbed both as haem and non-haem iron chiefly in the duodenum and the proximal jejunum. Iron in meat, primarily haem iron, is better absorbed than non-haem iron. The absorption of haem iron is not influenced by dietary composition or luminal factors as is the absorption of non-haem iron.
Breakdown of these controls could result in metal imbalance and metal overload, causing iron overloading toxicity and possibly death in many groups of patients, especially those with idiopathic haemochromatosis (see, for example, Guyader et al., in Gastroenterol., 97:737-743 (1989)). Shifting of immunoregulatory balances by iron excess or deficiency may produce severe, deleterious psychological effects.
Iron, particularly in the form of free iron ions, can promote the generation of reactive oxygen species through the iron-catalyzed Fenton and Haber-Weiss reactions (Haber and Weiss, in Proc. R. Soc. Ser. A 1934; 147:332) as follows:Fe3++.O2−→Fe2++O2 Fe2++H2O2→Fe3++OH−+.OH.O2−+H2O2→.OH+HO−+O2 
The Haber-Weiss and Fenton reactions are seen to produce the hydroxyl radical (.OH), a highly potent oxidant which is capable of causing oxidative damage to lipids, proteins, and nucleic acids (Lai and Piette. Biochem Biophys Res-Commun 1977; 78:51-9, and Dizdaroglu and Bergtold. Anal Biochem 1986; 156:182). See also FIG. 3 herein.
The effects of iron overload include decreased antibody-mediated and mitogen-stimulated phagocytosis by monocytes and macrophages, alterations in T-lymphocyte subsets, and modification of lymphocyte distribution in different compartments of the immune system. Accordingly, among its toxic effects, iron is known to mediate a series of oxygen related free radical reactions (see, for example, Halliwell and Gutteridge, in Halliwell and Gutteridge, Free Radicals in Biology and Medicine, 2nd edition. Oxford: Clarendon Press, 15-19 (1989)).
In particular, haemochromatosis is a disease of excessive iron storage leading to tissue damage and fibrosis. Both genetic, or hereditary, haemochromatosis, which can affect 1 in 500 of some populations, and the form of this disease which occurs as a secondary consequence of the haemoglobinopathy, homozygous β-thalassemia, with 40 million carriers worldwide, have a common pathology. Haemochromatosis of the liver in man is caused when the iron burden exceeds a threshold in the region of 22 μmol/g liver dry weight.
Genetic haemochromatosis, a life-long disease, is probably the most common autosomal recessive disorder found in white Americans, of who about 5/1,000 (0.5 percent) are homozygous for the associated gene. The haemochromatosis gene is probably located close to the HLA-A locus on the short arm of chromosome 6. Homozygous individuals may develop severe and potentially lethal haemochromatosis, especially after age 39.
Hereditary haemochromatosis involves an increased rate of iron absorption from the gut with subsequent progressive storage of iron in soft organs of the body. Excessive iron storage eventually produces pituitary, pancreatic, cardiac, spleen, epidermal, and liver and/or hepatic failure or cancer. Damage to these organs may be characterized by elevated liver enzyme values and hepatomegaly often with cirrhosis which may develop into hepatocellular carcinoma, splenomegaly, pancreatic fibrosis leading to diabetes mellitus, hyperpigmentation of the skin, pituitary insufficiency, hypogonadism, occasional hypothyroidism, cardiac abnormalities such as arrythmias and/or congestive heart failure, and arthritis/arthropathy. Early diagnosis can prevent these excess iron-induced problems. Iron overload owing to HLA-linked hereditary haemochromatosis can be distinguished from other causes of haemochromatosis by liver biopsies and interpretations.
Iron overload as seen in hereditary haemochromatosis patients enhances suppressor T-cell (CD8) numbers and activity, decreases the proliferative capacity, numbers, and activity of helper T cells (CD4) with changes in CD8/CD4 ratios, impairs the generation of cytotoxic T cells, and alters immunoglobulin secretion when compared to treated hereditary haemochromatosis patients or controls. A correlation has recently been found between low CD8+ lymphocyte numbers, liver damage associated with HCV positivity, and severity of iron overload in β-thalassemia major patients. Iron overload, with its associated increases of serum iron levels and transferrin saturation, may cause a poor response to interferon therapy. Iron overload with hyperferremia is associated with suppressed functions of the complement system (classic or alternative types).
The mesylate salt of desferrioxamine B (Desferal, CAS: 138-14-7; Novartis) is the first-line treatment for patients who suffer from transfusional-dependent iron-overload disease, which occurs as a secondary complication of β-thalassemia. Estimates suggest that 270 million people worldwide are carriers of β-thalassemia and that 200,000 babies are born with the disease each year.
In order to treat β-thalassemia, patients must undergo two or three blood transfusions per month. Since each unit of blood contains approximately 220 mg of iron, this transfusion regime results in an average daily iron intake of 15-22 mg/day, which is significantly in excess of the normal daily intake of 1 mg/day. Iron is able to readily gain access to highly vascularised organs, such as the heart, liver and the endocrine glands; without treatment, transfusional-dependent iron-overload that results from the increased haem catabolism can lead to death, most often from cardiomyopathy.
In order to ameliorate the excess Fe burden, patients with β-thalassemia undergo chelation therapy with Desferal. Desferal has a poor plasma half-life (t½˜15 min) due to a combination of poor distribution and rapid clearance—the drug is limited to distribution in the plasma and, therefore, no Desferal reservoir is able to be maintained in other cellular compartments. Desferal is not effective when administered orally (15% of an orally administered dose is absorbed [1]) and patients must undergo subcutaneous infusions with Desferal for up to 70 h per week. The intolerable treatment regime for iron-overload disease leads to poor patient compliance and can lead to premature death. Furthermore, if iron-chelation therapy is discontinued, patients usually die, usually from cardiotoxic events. For example, see FIG. 4 for typical survival curves for patients with thalassemia syndromes comparing various treatment regimens.
The motivation to find orally-available iron chelation agents has lead to the discoveries of deferiprone (L1) and deferasirox (ICL670, 4-[3,5-bis-(2-hydroxyphenyl)-[1,2,4]-triazol-1-yl]benzoic acid); these abiological chelates are used orally in some patients who are not able to take Desferal. While these compounds are orally available, both ICL670 and L1 are less effective than Desferal at binding Fe(III) and have poorer toxicity profiles. Several fatalities in the US from irreversible renal failure following treatment with ICL670 has made the future of this candidate uncertain. Clearly, there is a need for new compounds that are effective at treating iron-overload disease and that have tolerable administration regimes. The poor pharmacological profile of Desferal is largely attributed to the low membrane partition coefficient of the drug—Desferal is very water soluble and is inefficient at crossing the lipid membrane bilayer of cells. This affects the potential for DFOB to access intracellular iron pools which is one of major shortcomings in DFOB-based iron overload treatment.
Furthermore, the Desferal™ treatment regime is not well tolerated by patients since the drug is not substantially orally active and has a relatively short plasma half-life (approx. 15 min) and, therefore, must be administered subcutaneously or intravenously. Indeed, patients can spend up to 70 h per week connected up to a pump for administering chelation therapy (e.g. 14 hrs/day for 5 days/week). Apart from the inconvenience, this also leads to poor compliance in at least 50% of patients. It has been estimated that 200,000 babies are born with thalassaemia major each year. Prior to the development of Desferal™, these patients typically had life expectancies of less than 30 years (70% of all deaths in these patients is due to myocardial disease) and were dependent on life-long blood transfusions.
There still remains a need for an improved treatment of iron overload disease in humans. There is also a need for improved compounds and formulations for the treatment of iron overload disease in humans, and in particular, a need for orally available iron chelators and/or to find drug molecules with less arduous treatment regimes than Desferal™, since a more tolerable treatment regime leads to higher patient compliance and less mortality in patients. Preferably, such drug molecules may have superior pharmacokinetic properties to Desferal™.